So I have a nice NI 9209 input module and it lists the input impedance as > 1GΩ. But an impedance is COMPLEX so this can't be true. Any one worth their salt in using an instrument needs to know the input resistance AND the input capacitance. But NI claims to be a hardware company but gives out incomplete specifications on their hardware. This should be a universal part of any data sheet and a standard check for any one publishing data.
Why is this important? Well, I have a signal with both a high speed lower accuracy measurement and a low speed high accuracy measurement. These can be combined to give a nice composite measurement. However the low speed signal seems to drop a glitch into the high speed data. I assume this is a huge capacitance of the MUX on that unit. But how much capacitance? It is not like NI helpfully provides a circuit schematic (for various reasons). But it would certainly be helpful to know that capacitance in designing any sort of system of reasonable performance. Can an NI engineer look up that spec? Or look at the schematic and tell me that capacitance?
Further the module data sheet is missing the input impedance in the powered off mode which is a lot less than 1 GΩ! The data sheet should list the impedance for both powered and unpowered states.